For me I found Strathholme to be challenging. I was in a group and our tank constantly struggled to control the mobs. The healer struggled to keep him up, and if extra mob packs were pulled it often spelled doom for us. We wiped about 3 times and our tank had to tele in and out of the instance to repair. We tried CCing some of the healing mobs to prevent prolonged fights. The final boss Balnazzar took quite a bit to take out, and the healer had to keep the heals coming instead of alt tabbing.
Although it took longer, I grew a level and a half from the dungeon and quests, making it worthwhile time-wise.
Even though we couldn't race through it, it was still fun. I understand the healer and tank might not have been the best, but it was still fun. What other dungeons have you tried that have been a bit of a challenge?